1. Field of the Invention
The present invention relates to character projection units wherein different characters are selectively displayed on a common screen area. In particular, the present invention relates to improvements in the lens construction for such units.
2. Description of the Prior Art
In the past, character projection units have been employed using large multiples of the units to display multi-digit numbers, words or symbols. In one type of prior art unit, each projection unit incorporates a rectangular viewing screen which screen occupies a space in an overall panel display and with each space reserved for a single letter, digit or symbol. By selecting the particular number or letter projected by each unit, different multi-digit numbers and words may be displayed. Since each projection unit must be capable of displaying any of a relatively large selection of letters or numerals, the internal construction is complicated. A further complication is that the display units must be closely packed together so as to provide a conventional spacing between the digits and letters.
In another type of prior art device, the problem of compactness is solved by the employment of lens plates which support the individual lenses of a multiplicity of converging projection systems for superimposing the plurality of characters on a common display screen. Character projection units of this type may be seen by reference to prior U.S. Pat. Nos. 3,041,600 issued June 26, 1962 and 3,244,071 issued Apr. 5, 1966. In these prior art projection units the various lenses formed on the lens plates which together form the character projection units have these individual lenses formed with spherical surfaces. These spherical lens surfaces provide for a relatively sharp display of a character on the display screen if all of the elements in the projection system are properly aligned. However, with the prior art devices the spherical lens systems may produce a blurring of the image or a shadow on the screen unless all of the elements in the system are properly aligned.
In particular, the light sources are generally filament lamps and during manufacture of the lamps the filament may not be in a consistent position. With the prior art lens structures, a portion of the light projection may be outside of the aperture stop of the front projection lens because of the improper position of the filament. When this occurs, the image on the projection screen will not be clear. In the prior art it is therefore necessary to provide for an accurate assembly of the light sources so as to adjust the individual position of the lamps to make sure that the filaments are properly positioned so that the light projection falls within the aperture stop of the front projection lens. If the filament is not accurately adjusted the light projection would not be true and this could produce shadows or blurring of the image on the screen. The problem unfortunately continues even after the original assembly of the projection unit. Every time a lamp has to be replaced a number of lamps must be tried and adjusted in position to eliminate the above-described problem.